VIC: Model Overview

1. VIC: Model Overview Ted Bohn UW Surface Water Hydrology Seminar November 15, 2012

2. Outline • VIC Overview • VIC Web Site • How to Run VIC • How to Run the Routing Model • Future Development • Conclusions

3. VIC Model Overview What is VIC? • Large-scale hydrologic model (Liang et al, 1994) • Simulates water & energy storages and fluxes • Inputs: daily or sub-daily meteorological drivers • Precipitation • Air temperature • Wind speed • etc • Model time step length = daily or sub-daily

4. VIC Model Overview VIC is intended for • Large regions (> 100,100 km2) • Monthly flows and land-atmosphere fluxes • Coupling with GCMs

5. VIC Model Overview VIC Makes Some Approximations: • Land surface = grid of large (>1km), flat, uniform cells • Equations derived for small, uniform cases are applied to very large heterogeneous areas • Sub-grid heterogeneity is handled via statistical distributions • Elevation • Land cover • Water can only enter a grid cell via the atmosphere • Non-channel flow between grid cells is ignored • MOST runoff reaches local channel network before reaching grid cell boundary • Once water reaches the channel network, it is assumed to stay in the channel (it cannot flow back into the soil) • Grid cells are simulated independently of each other • Routing of stream flow is performed separately from the land surface simulation, using a separate model (Lohmann et al., 1996 and 1998)

6. VIC Model Features Validity of these assumptions? 1. Uniform grid cells/small-scale equations • Depends on: • Linearity of the processes (does average land surface produce average results?) • Statistical independence of processes (no correlations/interactions) 2. Runoff goes to local channels/ doesn’t come back/independence of grid cells • Depends on: • Grid cell size • Channel network density • Rule of thumb: Grid cell size > spacing between channels (or big enough to contain typical hill or mountain) • Groundwater flow must be small

7. VIC Model Features Meteorological Inputs • VIC can accept any combination of daily or sub-daily met. variables • At minimum, VIC requires daily Prcp, Tmin, Tmax, and Wind Speed • VIC uses MTCLIM algorithms to compute daily (and hourly) incoming shortwave radiation and daily humidity • VIC uses Prata algorithm to compute daily incoming longwave algorithm • VIC uses cubic spline to interpolate diurnal cycle of T • From diurnal T, computes sub-daily values of other forcings • Prcp and Wind held constant throughout day

8. VIC Model Features Land Cover • “Tiles” • Each type of vegetation gets 1 tile • Optional lake/wetland tile • Spatial distribution ignored • Each tile is homogeneous • Trees have same height, LAI, etc • Each tile modeled separately • Fluxes and storages from the tiles are averaged together (weighted by area fraction) to give grid-cell average for output

9. VIC Model Features Evapotranspiration (ET) • From canopy storage • From soil via transpiration OR bare soil evap • If veg present, no bare soil evap • If overstory, no understory evap 6 types of Potential Evap (PET) • Open water • Wet soil • Natural Veg with no soil moisture limitation (but other resistances intact) • Natural Veg with no environmental limitations • Short reference crop • Tall reference crop

10. VIC Model Features Penman-Monteith formulation = f(radiation, humidity, wind, surface roughness, architectural resistance, stomatal resistance) • Architectural Resistance • Shape of canopy; impedance to turbulent transport • = f(LAI, canopy structure) • Stomatal Resistance • Stomata: pores in leaves for transport of H2O, CO2 • Plants actively change stomatal opening to optimize photosynthesis and retain moisture • = f(radiation, temperature, humidity, soil moisture, LAI) • Evap of canopy storage (dew on leaves) • = Penman-Monteith with stomatal resistance set to 0 • Transpiration • = Penman-Monteith • Bare soil evap • = Penman-Monteith with architectural and stomatal resistance set to 0 • Only applied to saturated soil fraction

11. VIC Model Features Soil • Typically 3 soil layers • Infiltration into the top-most layers controlled by variable infiltration capacity (VIC) parameterization • Top-most layers can lose moisture to evapotranspiration • Gravity-driven flow from upper layers to lower layers • Q = f(soil moisture, Ksat) • ARNO baseflow formulation for drainage from bottom layer • Optional organic content in soil parameter file • Optional water table position computed via Van Genuchten curve

12. VIC Model Features Soil T profile • 1. QUICK_FLUX approximation • (QUICK_FLUX TRUE in global parameter file) • Assumes exponential decay towards “deep temperature” with depth • 2. Finite element scheme • (QUICK_FLUX FALSE in global parameter file) • Heat diffusion equation • Anywhere from 3 to 50 “nodes” • Hydro soil layer average T can be output (as of VIC 4.1.2) • Optional soil freeze/thaw: FROZEN_SOIL TRUE in global parameter file • Finite element + Frozen Soil is computationally expensive • Only recommended where soil freezing is important process

13. VIC Model Features Soil T heterogeneity • Option: SPATIAL_FROST (user_def.h) • Linear (uniform) distribution of soil T around mean • Allows some moisture movement in soil when avg T below freezing

14. VIC Model Features Snow • 2-layer formulation • Precip, Sublimation and melt • Albedo decay • Energy balance • Need diurnal radiation to get snowmelt rate right • SNOW_STEP in global parameter file • Partial snow cover can also be considered (affects melt rate) • (SPATIAL_SNOW TRUE in user_def.h)

15. VIC Model Features Elevation Bands • Snow pack size depends non-linearly on altitude, via precipitation and temperature • In temperate mountainous regions, with seasonally-varying snow line, using the average elevation, temperature, and precipitation over the entire grid cell gives wrong answer • How it works: • Subdivide grid cell into arbitrary number of elevation bands • Within each band, meteorologic forcings are lapsed from grid cell average elevation to band's elevation • Geographic locations or configurations of elevation bands are not considered; VIC lumps all areas of same elevation range into 1 band • Fluxes and storages from the bands are averaged together (weighted by area fraction) to give grid-cell average for writing to output files • Specify in global parameter file • Less necessary when running VIC at high resolution (1/16 degree)

16. VIC Model Features • Dynamic Lake/Wetland Model (Bowling, 2002, 2009) • Multi-layer lake model of Hostetler et al. 2000 • Energy-balance model • Mixing, radiation attenuation, variable ice cover • Dynamic lake area (taken from topography) allows seasonal inundation of adjacent wetlands • 1 composite lake per cell • Lakes can (as of VIC 4.1.2) receive input from upstream channel network: • Requires running upstream cells first • One option: run entire basin without lakes, then route flows, then simulate lake as single grid cell • Note: lake parameters (bathymetry, wfrac, mindepth, etc) not available on global scale; so far calibrating case-by-case

17. VIC Model Features River Routing • Routing of stream flow is performed separately from the land surface simulation, using a separate model, typically the routing model of Lohmann, et al. (1996; 1998) • Each grid cell is represented by a node in the channel network • The total runoff and baseflow from each grid cell is first convolved with a unit hydrograph representing the distribution of travel times of water from its points of origin to the channel network • Then, each grid cell's input into the channel network is routed through the channel using linearized St. Venant's equations • A lot of slop here: we typically don’t calibrate the unit hydrograph, channel velocities, etc. Errors are small when looking at monthly flows from large basins

18. Outline • VIC Overview • VIC Web Site • How to Run VIC • How to Run the Routing Model • Future Development • Conclusions

19. VIC Web Site www.hydro.washington.edu/Lettenmaier/Models/VIC/

20. Outline • VIC Overview • VIC Web Site • How to Run VIC • How to Run the Routing Model • Future Development • Conclusions

21. How to Run VIC • Download source code • Compile (via “make”) • Prepare input files • Run VIC vicNl –g global_parameter_file >& log.file.txt Other Usage: • To see version info: vicNl –v • To see compile-time option settings: vicNl –o

23. Global Parameter File Global Parameter File = Master Input File • Names/locations of all other input/output files • Contents/formatting of input forcing and output files • You can make whatever output files you want • Settings of most simulation options • Start/stop dates • Model time step • Water Balance v. Full Energy mode • Etc • Options and their allowed values are listed in vicNl_def.h

24. Other Parameter Files • ASCII text • 1+ line(s) per grid cell • Formats described on VIC web site • Optionally, soil parameters can be specified in Arc/INFO grid format

25. Met. Forcing Files • ASCII or binary • Can be traditional units (e.g. mm) or ALMA convention (e.g. m3/sec) • ALMA = standard for atmospheric models • You MUST describe file contents in the global parameter file • Variables • Start date • Time step • Measurement height • Usually 1 file per grid cell • Optionally a 2nd file • Can have different variables, time step, start date than first file • New input forcing variable, CHANNEL_IN, is only used for input into a lake • Can be the routed runoff of upstream cells

26. State Files • Can be ASCII or Binary • Initial state file has same format as final state file • Initial state file can be the final state file from a previous VIC run

27. Output Files • Can be ASCII or Binary • Units can be traditional or ALMA convention • 1+ file(s) per grid cell • You can either: • Specify the files’ names and contents in the global parameter file • Or omit this information and have VIC create default output files • You can either: • Specify the output time step (n-hourly or daily) • Or omit this and the output time step will = model time step

28. VIC Simulation Mode Settings 2 types of settings: • Run-time • Most options are of this type • Set in global parameter file • Examples of most available in global.param.sample • Compile-time • Only a few options • Set in user_def.h • Values in user_def.h may be misleading, if it has been edited since the last “make” • To see their current values, type vicNl –o • To change these: • Edit user_def.h • Make clean • Make

29. vicNl_def.h Describes Most Model Options • Input Forcing Variables /***** Forcing Variable Types *****/ #define N_FORCING_TYPES 24 #define AIR_TEMP 0 /* air temperature per time step [C] (ALMA_INPUT: [K]) */ #define ALBEDO 1 /* surface albedo [fraction] */ #define CHANNEL_IN 2 /* incoming channel flow [m3] (ALMA_INPUT: [m3/s]) */ • Output Forcing Variables /***** Output Variable Types *****/ #define N_OUTVAR_TYPES 160 // Water Balance Terms - state variables #define OUT_ASAT 0 /* Saturated Area Fraction */ #define OUT_LAKE_AREA_FRAC 1 /* lake surface area as fraction of the grid cell area [fraction] */ #define OUT_LAKE_DEPTH 2 /* lake depth (distance between surface and deepest point) [m] */

30. vicNl_def.h Describes Most Model Options • Simulation Modes: options_struct typedef struct { // simulation modes int AboveTreelineVeg; /* Default veg type to use above treeline; Negative number indicates bare soil. */ char AERO_RESIST_CANSNOW; /* "AR_406" = multiply aerodynamic resistance … } option_struct

31. Example global parameter file • Included in code distribution • To make global parameter file for your project, copy this file and edit it • Contains most options, set to default values Global.param.sample ####################################################################### # VIC Model Parameters - 4.1.x ####################################################################### # $Id: global.param.sample,v 5.7.2.28 2012/04/15 05:39:10 vicadmin Exp $ ####################################################################### # Simulation Parameters ####################################################################### NLAYER 3 # number of soil layers NODES 10 # number of soil thermal nodes TIME_STEP 3 # model time step in hours (set to 24 if FULL_ENERGY = FALSE, set to < 24 if FULL_ENERGY = TRUE) SNOW_STEP 3 # time step in hours for which to solve the snow model (should = TIME_STEP if TIME_STEP < 24) STARTYEAR 2000 # year model simulation starts STARTMONTH 01 # month model simulation starts STARTDAY 01 # day model simulation starts STARTHOUR 00 # hour model simulation starts ENDYEAR 2000 # year model simulation ends ENDMONTH 12 # month model simulation ends ENDDAY 31 # day model simulation ends FULL_ENERGY TRUE # TRUE = calculate full energy balance; FALSE = compute water balance only FROZEN_SOIL TRUE # TRUE = calculate frozen soils ####################################################################### # Soil Temperature Parameters

32. Outline • VIC Overview • VIC Web Site • How to Run VIC • How to Run the Routing Model • Future Development • Conclusions

33. How to Run the Routing Model • Download source code • Compile (via “make”) • Prepare input files • Run rout rout input_file >& log.file.txt

35. Input Files “Input” File = Master Input File • Names/locations of all other input/output files • Settings of simulation options • Start/stop dates • Etc Unit Hydrograph File • Distribution of overland travel times for a pulse of runoff to reach the channel (impulse response) • ASCII text • Format: time(h) flow_fraction Station Location File • ASCII text • List row,column coordinates of station(s) for which you want a hydrograph

36. Input Files Flow Direction File • ASCII Arc/INFO • Numeric code indicates pixel’s flow direction Flow Fraction File • ASCII Arc/INFO • Fraction of pixel’s runoff that enters channel network • Usually = fraction of pixel that’s in the basin

37. Input Files Other Optional ASCII Arc/INFO Files: (You can either specify a filename or just a single value that will be used in all pixels) • Flow Velocity File • Diffusivity File

38. Input Files Runoff Files • ASCII Text (or NetCDF, in surface water monitor) • ASCII: require the following format Year Month Day (data) (data) Runoff Baseflow • Extra columns at the end of the line are allowed (and ignored) • VIC’s default output “fluxes” files have this format • Daily runoff and baseflow will be added together

39. Output Files Daily, Monthly, Annual flow files Units depend on settings in main input file

40. Outline • VIC Overview • VIC Web Site • How to Run VIC • How to Run the Routing Model • Future Development • Conclusions

41. Future Development Possible additions: • NetCDF Input/Output • Complete Carbon Cycle (port from my work) • Lakebottom heat flux (port from my work) • Distrib water table (port from my work) • Methane? • More user-friendly lake model • Multiple soil types per cell • Replace (veg tile, snow band) array with “patches”, each with own soil, veg, elevation, etc characteristics • Flow routing within VIC • Suggestions?

42. Large-Scale Modeling: Challenges How to Validate Model Results? • Large-scale simulations vs. point measurements • Soil moisture extremely heterogeneous • One point is NOT representative of entire grid cell • Streamflow is our friend! • Aggregates the runoff of the entire basin above the gauge • How would we do this with other hydrologic variables? • Satellites can measure things visible at surface • Problem: lateral extent (e.g. snow cover) not equal to total storage (e.g. snow depth)

43. Conclusions • Goals of large-scale models not the same as hillslope-scale models • Different assumptions & approximations • Large-scale behavior may look different from small-scale • Non-linearities • Interactions • Fine-scale variations, where important, represented by statistical distributions • Validation of large-scale models requires large-scale observations, special techniques

44. Thank You • More information at VIC web site: www.hydro.washington.edu/Lettenmaier/Models/VIC/

45. Large-Scale Modeling: Overview What is a model? • (Idealized) Representation of some aspect of natural world • Aid to understanding the real world • (Usually) Simpler than the real world • Captures most important features of the system • Less important features may be omitted or approximated

46. Large-Scale Modeling: Overview Types of Models: • Numerical (computer) • Analytical (equations) • Penman-Monteith Transpiration • Newton’s Gravity • Conceptual (need not be mathematical) • Role model • Fashion model • Model airplane • Etc

47. VIC = Large-Scale Land Surface Hydrology Model • Captures important features of largehydrologic systems • Sub-continental to global systems • >100,000 km2 • Coarse resolution (why?) • Spatial: 1/16- to 2-degree (5-200 km) • Temporal: generally analyze outputs at monthly scales • Inputs • Meteorology • Soil, vegetation, topography • Outputs • Water & energy storages and fluxes (snowpack, streamflow, soil moisture, etc)

48. Large-Scale Modeling: Overview Why use a large-scale hydrologic model? • Estimating continental/global: • water budgets • land-atmosphere energy fluxes • Allocation of international water resources • Drinking water • Agriculture • Hydropower • Environmental • Hydrologic impacts of large-scale climate variations/change • El Nino, Pacific Decadal Oscillation, etc • Global Warming (These all have signatures that vary over large spatial scales) • Land surface component of global climate model

49. Large-Scale Modeling: Overview Why Coarse Resolution? • Computational Efficiency • Large-scale questions don’t require fine resolution • Uncertainties at fine resolution • Observations • Model Equations

50. Large-Scale Modeling: Overview Primary Analysis Needs: • Availability of water resources • River discharge • Soil moisture • Snow pack & Reservoir storage • Large areas • Historical and projected • Seasonal averages • Interannual variability & trends Generally, coarse outputs (monthly data, at 1/8-degree or larger resolution), are sufficient • Room for slop…